recuperative braking. For example, an electric drive motor of the vehicle is driven by a generator, the electrical energy thereby generated being fed into a storage device, in particular a battery, and then being retrievable as needed. The vehicle's power loss is reduced by recuperative braking, which is thus a suitable measure for reducing consumption and emissions.
Recuperative braking makes high demands on the cooperation of the components of the braking system which generate a friction-based braking torque, and components which generate a recuperative braking torque, because the recuperative braking torque depends on several factors: for example, a recuperative braking torque is not available when the battery is fully charged, so that the entire braking torque must be applied by friction-based braking. In addition, the recuperative braking torque depends on the rotational speed of the generator and thus on the speed of the vehicle, which is why it is not constant. For example, virtually no recuperative braking torque is available at low rotational speeds. Furthermore, even if recuperative braking is impossible, this must not result in a longer braking distance due to a component failure, for example.
Either the recuperative braking torque may be used in addition to the existing friction-based braking torque or the recuperative braking torque and the friction-based braking torque are controlled in such a way that their sum always corresponds to the driver's instantaneous braking intent. The latter approach is known as CRBS (cooperative regenerative braking system).
Various approaches are available for implementing CRBS in combination with pedal force simulators and power braking systems. These ensure complete or partial decoupling of the reactive forces acting on the driver via the brake pedal, which result from the pressure changes (brake pressure modulations) in the braking system associated with recuperation. In particular, however, in the case when CRBS is to be used in conjunction with traditional brake boosters, for example, a vacuum brake booster, it is much more difficult to control the pressure in the braking system, so that the driver does not perceive any reactions on the brake pedal during recuperative operation of the braking system. It may be a problem that reactions may make the driver feel insecure.
German Patent Application No. DE 10 2007 030 441 A1 describes a device having a cylinder, a piston which is movable in the cylinder for receiving and dispensing hydraulic fluid and a drive unit in the form of an electric motor. The electric motor is connected to the piston for movement of the latter with the aid of a threaded spindle. This device has the function of providing a constant pedal perception on the brake pedal to the driver of a hybrid or electric vehicle, regardless of whether or not recuperation is taking place.